Liquid Water Content Values Research Articles

A numerical study of aerosol effects on the dynamics and microphysics of a deep convective cloud in a continental environment

The effects of aerosols on a deep convective cloud in a midlatitude continental environment are studied using an axisymmetric cloud model with a sectional treatment of aerosol and hydrometeor microphysical processes. Simulations are conducted using observations from the Cooperative Convective Precipitation Experiments (CCOPE). The isolated cloud occurred in an environment with low wind shear and with relatively dry air in the midtroposphere and upper troposphere. By varying the concentration of aerosol particles in the accumulation mode within realistic limits for a continental environment, the simulated cloud exhibited different properties. The overall impact as the aerosol concentration increased is that (1) the cloud development was inhibited; (2) the precipitation was suppressed; (3) the maximum values of liquid water content decreased, but the maximum values of droplet number concentration increased before the dissipating stage; (4) a clear tendency was found for ice crystals to be larger and less numerous in the anvil cloud; and (5) there was a significant reduction of the inflow in the lower 2 km of the atmosphere. In the relatively dry environment in the midtroposphere, the latent heat changes associated with the Wegener‐Bergeron‐Findeisen mechanism played an important role in the upper part of the cloud at altitudes below the homogeneous freezing level. In particular, immersion freezing and latent heat release were much more rapid in the base simulation than in the increased aerosol simulation. Less latent heat release and insufficient inflow together impeded the development of the cloud with the higher aerosol loading. Our simulations suggest that continental clouds existing below the homogeneous freezing level could show an opposite response of cloud top height and anvil crystal concentrations to changes in aerosol to what has previously been reported for clouds ascending to higher levels.

Calculated and measured values of liquid water content in clean and polluted environments

The relationship between liquid water content (LWC) and visibility (VIS) in a fog was examined by a field measurement of LWC values at a clean location of the meteorological observatory Sodankyla (Finland) and in a polluted region at the meteorological observatory Milesovka (Czech Republic). Furthermore, the use of a simple regression fog model to determine LWC from VIS is examined by comparing well-known relationships with the measurement results.

Turbulence as a Mechanism for Orographic Precipitation Enhancement

Abstract This study examines the dynamical and microphysical mechanisms that enhance precipitation during the passage of winter midlatitude systems over mountain ranges. The study uses data obtained over the Oregon Cascade Mountains during the Improvement of Microphysical Parameterization through Observational Verification Experiment 2 (IMPROVE-2; November–December 2001) and over the Alps in the Mesoscale Alpine Program (MAP; September–November 1999). Polarimetric scanning and vertically pointing S-band Doppler radar data suggest that turbulence contributed to the orographic enhancement of the precipitation associated with fronts passing over the mountain barriers. Cells of strong upward air motion (>2 m s−1) occurred in a layer just above the melting layer while the frontal precipitation systems passed over the mountain ranges. Upstream flow appeared to be generally stable except for some weak conditional instability at low levels in the two IMPROVE-2 cases. The cells occurred in a layer of strong shear at the top of a low-level layer of apparently retarded or blocked flow (shown by Doppler radial velocity data). The shear apparently provided a favorable environment for the turbulent cells to develop. The updraft cells appeared at the times and locations where the shear was strongest (>∼10 m s−1 km−1). The Richardson number was slightly less than 0.25 at the level where the cells were observed, suggesting shear-generated turbulence could have been the origin of the updraft cells. Another possibility is that the rough mountainous lower boundary could have triggered buoyancy oscillations within the stable, sheared flow. The existence of turbulent cells made possible a precipitation growth mechanism that would not have been present in a laminar upslope flow. The turbulent cells appeared to facilitate the rapid growth and fallout of condensate generated over the lower windward slopes of the mountains. In a laminar flow over terrain, upward motions would be unlikely to produce liquid water contents adequate to increase the density (and hence the fall speed) of precipitating ice particles by riming. The turbulent updraft cells apparently create pockets of higher values of liquid water content embedded in the widespread frontal cloud system, and snow particles falling from the parent cloud systems can then rapidly rime within the cells and fall out. Observations by polarimetric radar and direct aircraft sampling indicate the occurrence of rimed aggregate snowflakes and/or graupel in the turbulent layer. Inasmuch as the shear layer is the consequence of retardation or blocking of the low-level cross-barrier flow, and the turbulence is a response to the shear, the shear-induced cellularity is an indirect response of the flow to the topography. The turbulence embodied in this orographically induced cellularity allows a quick response of the precipitation fallout to the orography since aggregation and riming of ice particles in the turbulent layer produce heavier, more rapidly falling precipitation particles. Without the turbulent cells, condensate would more likely be advected farther up and perhaps even over the mountain range. Small-scale cellularity has traditionally been associated with the release of buoyant instability by the upslope flow. Our results suggest that cellularity may be achieved even if buoyant instability is weak or nonexistent, so that even a stable flow has the capacity to form cells that will enhance the precipitation fallout over the windward slopes.

A study of thermals in cumulus clouds

Air motions in the thermals contained within shallow Florida cumulus clouds were observed to be similar to the circulation observed in laboratory thermals. There was outward flow in the updraughts of individual thermals at most levels and there were usually downdraughts observed at the edges of the updraught or of the cloud. Widespread inward flow towards the centre of the cloud and a narrow, but strong, updraught was occasionally observed, reminiscent of the tail found at the rear of the laboratory thermals. A region of reduced liquid water content was frequently observed in the centre of several thermals where the updraught and horizontal 1D divergence were strongest, and complete holes were observed on two occasions. Although horizontal wind shear was generally weak, it was significant in a few cases, causing the flow pattern in the cloud to be asymmetric. Ascending regions of cloud with high values of liquid water content (cloud cores) were commonly observed at all altitudes, but generally the percentage of clouds measured with high liquid water content decreased with altitude. The observations of airflow and liquid water content structure in warm cumulus clouds described in this paper are consistent with the schematic model of a thermal where a core of high liquid water content survives for several kilometres above cloud base, but erodes as it ascends. Copyright © 2005 Royal Meteorological Society

Evaluation of Radar Reflectivity–Based Estimates of Water Content in Stratiform Marine Clouds

Abstract The performance of radar reflectivity (Ze)–based relations for retrievals of marine stratiform cloud liquid water content (LWC) is evaluated by comparing liquid water path (LWP) estimates from microwave radiometers with vertically integrated LWC values retrieved from radar measurements. Based on a measurement dataset from a research vessel in the tropical eastern Pacific Ocean, it is shown that reflectivity thresholding allows minimizing of the influence of drizzle drops present in marine stratiform clouds to the extent that LWP estimates from a ground-/shipborne radar can have uncertainties that might be acceptable for different applications. The accuracies of Ze-based retrievals depend on the thresholding level Zet, and they are generally better than a factor of 2 for Zet ≲ −15 dBZ. These accuracies typically improve when Zet is lowered; however, the amount of cloud profiles that pass thresholding diminishes as Zet is decreased from about 50% for a −15-dbZ threshold to only about 10% for a −25-...

Wind Tunnel Measurements of the Response of Hot-Wire Liquid Water Content Instruments to Large Droplets

Abstract Wet wind tunnel tests were performed on more than 23 cloud liquid water content (LWC) probes and drop spectrometers at the NASA Icing Research Tunnel, with a main objective to characterize their response to large-droplet conditions. As a part of this study, the LWC and median volume diameter (MVD) reference values of the tunnel were examined, and accuracies were estimated and reported herein. Alternative MVDs were calculated from measurements conducted during the study. MVD accuracy was particularly difficult to estimate due to the lack of accepted standards, and MVD estimates were quite sensitive to the complement of instruments used to generate composite droplet spectra. Four hot-wire LWC probes were tested to characterize the LWC response as a function of cloud MVD. A response reduction was observed with increasing MVD for all three probes with cylindrical hot wires, most significant for the probe with the smallest wire diameter. The response of the Nevzorov total water content (TWC) probe, wi...

Droplet Spectra Broadening in Cumulus Clouds. Part I: Broadening in Adiabatic Cores

Measurements of cloud droplet spectra performed with the Fast-Forward Scattering Spectrometer Probe during the Small Cumulus Microphysics Study (1995) are analyzed. Fifty cloud samples with narrow droplet spectra are selected. They are characterized by values of liquid water content slightly below the adiabatic value. Each observed spectrum is then compared to a narrow adiabatic spectrum predicted at the same level with the current theory of condensational growth in an adiabatic cloud cell, initialized with a reference spectrum measured right above the activation level, at cloud base. Broadening is characterized for each observed spectrum by the probability density function of condensational growth expressed as the Lagrangian integral of the ratio of supersaturation to vertical velocity, along the droplet trajectories. In particular it appears that the derived density functions show high probabilities of very low and very large values of condensational growth. The large values are related to a high relative density of big droplets in the measured spectra, higher than predicted by the adiabatic model. The contribution of the instrument to this feature is examined with a model of probe functioning. The simulations suggest that most of those big droplets are instrumental artifacts. The remaining broadening is parameterized by a linear relationship between the mean value and the standard deviation of the density function of condensational growth. This result will be used to examine the respective contributions to spectra broadening of microscale heterogeneities of the droplet concentration, in Part II, and of the mixing processes, in Part III of this series.

Observed extinction by clouds at 95 GHz

Measurements of backscattered power were made in maritime stratus clouds with a 95 GHz radar mounted on an aircraft. The aircraft also carried probes for in situ cloud measurements of liquid water content (LWC) and hydrometeor size distribution. Horizontal flight segments of 5-10 km length, over which the LWC was relatively uniform, were selected for analysis. Mean values of LWC ranged from 0.05 to 0.8 g/m/sup 3/. The observations yielded an extinction coefficient of 4.6 dB/km per g/m/sup 3/ of cloud water.

Retrieval of cloud base heights from passive microwave and cloud top temperature data

Water vapor profiling algorithms that treat liquid clouds explicitly yield a cloud base height as a byproduct. A single case of a water vapor profile retrieval using a combination of the SSM/T-2 on the DMSP satellite and cloud parameters from the AVHRR on the NOAA satellite retrieved a reasonable cloud base. While hardly definitive, this case is suggestive. The authors examine the cloud base signal in a combination of the SSM/1 and SSM/T-2 on the DMSP satellite from a theoretical point of view. It is shown that the signal is strong enough for a useful retrieval only over the ocean. For low altitudes, a cloud top temperature (CTT) constraint, as could be provided from an infrared radiometer, is required. While difficult with the DMSP-NOAA satellite combination, this has become much easier with the recent launch of NOAA-K with the AMSU-B and AVHRR. It is shown that the signal is acceptable over the relevant range of cloud liquid water content values. To achieve useful results, some local tuning of the algorithm will be necessary. This tuning could take the form of water vapor profile covariance matrices, climatological estimates of the cloud liquid water density, or purely empirical methods. Broken and multilayer clouds provide additional complications to the problem.

Ground-Based FSSP and PVM Measurements of Liquid Water Content

Abstract Recently published ground-based measurements of liquid water content (LWC) measured in fogs by two microphysical instruments, the FSSP-100 and PVM-100, are evaluated. These publications had suggested that the PVM-100 underestimated LWC significantly in comparison to the FSSP-100 when the fog droplets were large. The present evaluation suggests just the opposite: The FSSP-100 overestimates LWC for large droplets because these droplets are unable to follow the curved streamlines of the flow generated by drawing air into the FSSP-100’s sensitive volume at 25 m s−1. This inertial effect causes droplets to accumulate near the active volume of the instrument’s laser beam and to produce large and spurious droplet concentration and LWC values for the largest droplets. Model calculations estimate the magnitude of this error for the FSSP-100.

A comparison of optical measurements of liquid water content and drop size distribution in adiabatic regions of Florida cumuli

Airborne measurements of liquid water content (LWC) and drop size distribution were made in adiabatic regions of small, growing cumulus clouds during the Small Cumulus Microphysics Study (SCMS). A new instrument, the cloud drop spectrometer (CDS), which measures LWC and also drop size from an ensemble of drops, was flown for the first time in the field. Measurements from other sensors, including a Particle Measuring Systems (PMS) forward scattering spectrometer probe (FSSP), the `fast' FSSP (FFSSP) developed by the Centre National De Recherches Meteorologiques (CNRM), and a Gerber Scientific airborne particulate volume monitor (PVM-100A), are compared with the CDS data collected in adiabatic and other regions. The CDS appeared to reliably measure very close to the predicted value of LWC in regions identified as being adiabatic. In addition, the drop size distribution measured by the CDS compared very well with the FSSP and FFSSP measurements, except where the 3–200 μm range of the CDS allowed it to measure larger drops than the nominal 3–45 μm range of the FSSP, and the 2.7 to 38.4 μm range of the FFSSP.

The Potential of a Spaceborne Cloud Radar for the Detection of Stratocumulus Clouds

Abstract The radar reflectivity and liquid water content of stratocumulus clouds have been computed from cloud droplet spectra recorded during more than 4000 km of cloud penetrations by an aircraft, and the probability of detecting various values of liquid water content as a function of the radar sensitivity threshold has been derived. The goal of the study is to specify the sensitivity required for any future spaceborne cloud radar. In extensive marine stratocumulus deeper than about 200 m, occasional but ubiquitous drizzle-sized droplets of up to 200 μm dominate the radar return and increase it by between 10 and 20 dB above the cloud droplet contribution to the return, making radar detection easier, although the concentration of the drizzle drops is so low that they have no effect on the liquid water content or effective radius. These occasional drizzle-sized droplets are present throughout the vertical and horizontal extent of such clouds but should evaporate within 200 m of cloud base. On occasion, the drizzle can fall farther and may yield a false measurement of cloud-base altitude, but such cases can be recognized by examining the vertical profile of reflectivity. A radar sensitivity threshold of −30 dBZ would detect 80%, 85%, and 90% of the marine stratocumulus, with a liquid water content above 0.025, 0.05, and 0.075 g m−3, respectively. Because nonprecipitating drizzle droplets are rare in continental stratocumulus, the equivalent figures are reduced to 38%, 33%, and 25%. Improving the sensitivity to −40 dBZ increases detection probability to nearly 100% for both types of cloud. These figures are based on the assumption that the cloud is deep enough to fill the radar pulse volume.

Liquid Water Content and Temperature Relationship from Aircraft Observations and Its Applicability to GCMs

The vertical distribution of liquid water content (LWC) and its relationship with temperature (T) strongly affect the heat budget of the atmosphere. Some large-scale models of the atmosphere use a relationship between LWC and T to diagnostically obtain LWC from T under saturated conditions. Airborne observations conducted within clouds over northeastern North America during the 1984‐93 time period are used to study the relationship between LWC and T. Observed frequency distributions of LWC are approximated by lognormal distribution curves and are best represented by median values. The median LWC values monotonically increase with warmer temperatures. However, the mean LWC reaches 0.23 g m23 at about T 5 2.58C. LWC decreases below and above 2.58C, except that it reaches a maximum value of 0.26 g m23 at 22.58C. The relationship between LWC and T from the present study is compared with that of earlier studies from the former Soviet Union. Differences can be attributed to the design and limits of the probes, natural variability in the 35 years, and the limited dataset for some temperature intervals. The LWC versus T relationship developed from observations in this study can be compared with large-scale model simulations.

On the Caltech Active Strand Cloudwater Collectors

A detailed analysis of several versions of the Caltech Active Strand Cloudwater Collector (CASCC) is conducted. Efficiency calculations, design considerations and procedures for cloud liquid water content estimation from the collection rates of these instruments are discussed. The size-fractionating CASCC is capable of simultaneous collection of samples representing two portions of the cloud drop size spectrum. Large drops are collected in an inlet stage while smaller drops are collected in a second stage. Theoretical calculations, which assume no aerodynamic interaction between adjacent rows of collection rods in the inlet, suggest the inlet should have a 50% size cut corresponding to a drop size of 23 μm diameter. However, field test results suggest that focusing of the flow passing through a row of cylinders may increase the efficiency of collection on the subsequent cylinder row, thereby decreasing the overall size cut for the inlet. The CASCC2, a compact version of the original CASCC, is designed to sample the entire cloud drop spectrum. Comparison of the cloudwater collection rates of the CASCC2 and the size-fractionating CASCC showed good agreement when normalized by the flow rate through each collector. The Caltech Heated Rod Cloudwater Collector (CHRCC), designed for use in supercooled clouds, features a theoretical 50% lower size cut corresponding to a drop diameter of 9 μm. Liquid water content values estimated from the CHRCC cloudwater collection rates correlated reasonably well with values measured with a Gerber Particle Volume Monitor (PVM-100) in both warm ( r 2 = 0.83) and supercooled ( r 2 = 0.71) cloud conditions.

Thunderstorm Electrification Analysis: The Dependence on the Temperature–LWC Diagram

Abstract The dependence on liquid water content (LWC) and temperature (T) of the charge transfer, and in particular its sign, during ice crystal–graupel collisions is a matter of controversy. Some of the laboratory measurements do not agree on the general shape of this function, although they agree on the existence of a critical temperature Tr, above which there is one sign and below which the opposite one. In this paper, the authors carry out a numerical study to investigate the behavior of three charging parameterizations in thunderstorm electrification. This study is carried out on one cloud case study, and a limited set of T–LWC values are present. Surprisingly, the general behavior is the same among the three, despite the distinct differences in the charging diagrams.

CLOUD DROP SPECTRA IN MARITIME AND INLAND REGIONS

Measurements o f clo ud drop ..ire spectra on non-preci pitating cumulus clouds, 1·2 km thick,were made at differe nt levels o ver the Arabian Sea (maritime) and over r une (inland) regio n during the end ofthe summer monsoo n seasons of 1973, 1974 and 1979.The macimurn size of clo ud drops for the. Arabian Sea generally increased with height. while that for run eJiJ not show a systematic change with height. At bot h the local ions. the total concentration o f drops decreasedwith height. The maritime dist ributions were bimodal at all levels while those o..-er Punc were usually unimodal.The average values of liquid water content. mean vo lume diameter. dispersion and conccnt rarion of drops withdiameter > 50 «m were a lin le greater and total concentration a little smaller over the maritime region as ' om'pared to those over inland. The co ncentrations of drops with diameter < 14 ,...m and those > 78 I'm and thema ximum ~ i 7e were greater over Pune than o..er the sea. The ..'a riarions in cloud drop spectra and the clo udphysical parameters over beth the locations are discussed.Kc) words - Cloud drop size spectra, Drop co ncentration,

Collecting Supercooled Cloud Droplets as a Function of Droplet Size

Abstract Supercooled cloud droplets were inertially impacted onto “cloud-sieves” at a mountaintop location. The large cross-sectional areas of the sieve meshes permitted grams of cloud water to be passively collected in minutes. Each sieve was constructed from specific diameter cylindrical strands and collected all cloud droplets larger than a critical size. Procedures are developed to produce liquid water content (LWC) and chemical composition values as a function of droplet-size interval. The sieve LWC measurements were compared with simultaneous LWC measurements obtained from a standard cloud droplet spectrometer. The sieve and spectrometer values were consistent for droplets between approximately 4 and 13 µm in diameter. The sieves overestimated the water contents of larger and smaller droplets in low LWC clouds (<0.1 gm−3). In high LWC clouds, the sieve LWC values for all droplet sizes closely approximated the spectrometer values. Sources of error were investigated. Rime “feathers” and frost grew on ...

The effect of liquid water on thunderstorm charging

Laboratory studies have shown that thunderstorm charging caused by the interactions of ice crystals and graupel pellets is affected in sign and magnitude by temperature and cloud liquid water content; the presence of water droplets is a requirement for substantial charge transfer. Relationships showing the dependence of charge transfer on ice crystal size and velocity have previously been reported and now, in a continuation of the laboratory studies, the effect of liquid water content on the charge transfer has been investigated. The experiments show that positive graupel charging occurs at temperatures above a “charge sign reversal temperature” with negative charging at lower temperatures. The reversal temperature moves to lower temperatures when the liquid water content is increased. However, at low values of liquid water content, the sign of the graupel charging is inverted being positive at low temperatures and negative at higher temperatures. A discussion is presented of the various charge transfer theories. The results are consistent with the idea of two competing mechanisms whose relative success depends on the temperature and liquid water content. Positive graupel charging occurs when the graupel surface grows from the vapor and the crystals interact with a negative surface charge caused by a temperature gradient across the rime ice surface layer. Negative graupel charging occurs when the surface growth effect is swamped by freezing droplets which create either a pseudo contact potential with which the crystals interact, or a positive surface charge, due to dislocations in the rime ice, which is removed during glancing crystal interactions. Relationships between charge transfer, liquid water content, temperature, ice crystal size, and velocity have been determined and the equations may be used in numerical models of the development of thunderstorm electric fields. A one‐dimensional model indicates that the charge separation rates noted here are adequate to account for thunderstorm electrification. Use of the equations with cloud parameter values obtained in a thunderstorm research flight, leads to a predicted charge reversal level around −13°C, which is in agreement with the analysis of the electric field in the thunderstorm studied.

Size-resolved measurements of cloud droplet deposition velocity to a forest canopy using an eddy correlation technique

Two case studies are presented of the direct measurement of cloud droplet fluxes to a forest canopy by the eddy correlation technique. Two instruments have been used for the liquid water measurements, and close agreement for values of liquid water content and liquid water flux have been obtained. The case studies were conducted in different weather conditions: case 1 with high values of liquid water content (typically 0.3 g m−3) and low wind speeds (2–3 m s−1), and case 2 with low liquid water content (0.1 g m−3) and higher wind speeds (typically 6 m s−1). In both cases the deposition velocity for liquid water was markedly less than for momentum. For case study 2 the cause of the small deposition velocity for liquid water was the small droplet size; the typical modal radius was 4.4 μm. The deposition velocity increased as a function of droplet size, exceeding that for momentum for droplets larger than about 6 μm. When normalized, the results agreed reasonably well with those obtained over grassland using the gradient technique. The results are compared with the predictions of the models of both Lovett and Slinn.

Size‐resolved measurements of cloud droplet deposition velocity to a forest canopy using an eddy correlation technique

AbstractTwo case studies are presented of the direct measurement of cloud droplet fluxes to a forest canopy by the eddy correlation technique. Two instruments have been used for the liquid water measurements, and close agreement for values of liquid water content and liquid water flux have been obtained. The case studies were conducted in different weather conditions: case 1 with high values of liquid water content (typically 0.3 g m−3) and low wind speeds (2–3 m s−1), and case 2 with low liquid water content (0.1 g m−3) and higher wind speeds (typically 6 m s−1). In both cases the deposition velocity for liquid water was markedly less than for momentum. For case study 2 the cause of the small deposition velocity for liquid water was the small droplet size; the typical modal radius was 4.4 μm. The deposition velocity increased as a function of droplet size, exceeding that for momentum for droplets larger than about 6 μm. When normalized, the results agreed reasonably well with those obtained over grassland using the gradient technique. The results are compared with the predictions of the models of both Lovett and Slinn.